The production of hydrocarbons, such as oil and gas, has been performed for many years. To produce these hydrocarbons, one or more wells in a field are drilled to a subsurface location which is generally referred to as a subterranean formation or basin. The process of producing hydrocarbons from the subsurface location typically involves various development phases from a concept selection phase to a production phase. One of the development phases involves the drilling operations that form a fluid conduit from the surface to the subsurface location. The drilling operations may involve using different equipment, such as hydraulic systems, drill pipe, drill bits, mud motors, etc., which are utilized to drill to a target depth.
Mechanical specific energy (MSE) or specific energy (SE) is a measure of the energy required for a drilling assembly to drill through an interval of subterranean formation. The standard formula for determining MSE was originally derived by Teale (Teale, R, “The concept of specific energy in rock drilling,” Int. J. Rock Mech. Mining Sci., 1965; 2: 57-73) and may be evaluated using surface data only. Generalizations of this formula have been developed and applied over the last several decades in the oil and gas industry and, depending upon the adopted formulation, may be used in conjunction with surface and/or down hole data.
Under efficient drilling conditions, MSE correlates with the compressive rock strength of the subterranean formation. When a dysfunction, or limiter, is present in the drilling system, however, this correlation breaks down and MSE tends to increase beyond the rock's compressive strength. Common forms of drilling dysfunction (or drilling performance limiters) that can lead to such a deviation from efficient drilling conditions include mechanical vibrations (e.g., whirl, stick-slip and bit bounce, among others), bit balling, insufficient hole cleaning and drill string buckling, among others.
A common industry practice for seeking to avoid or minimize drilling performance limiters is to drill by MSE, or in other words to drill using operational set points, e.g., combinations of weight on bit, rotary speed and mud flow rate, that result in a minimum MSE while maintaining a forward rate of penetration (ROP). Drilling by MSE balances the desire for increased ROP with the need to avoid wearing out the bit and tools prematurely due to limiters. For example, U.S. Pat. No. 7,857,047 to Remmert et al. discloses a method of drilling and producing hydrocarbons from subsurface formations. The method includes performing drilling operations at one or more wells to a subsurface location in a field to provide fluid flow paths for hydrocarbons to a production facility. The drilling is performed by obtaining MSE data and other measured data during the drilling operations. The MSE data and other measured data may be used to determine the existence of at least one limiter. Lithology data for the well is obtained and examined. A primary limiter of the at least one limiter is identified based on the lithology data, and drilling operations are adjusted to mitigate at least one of the at least one limiter.
As another example, U.S. Pat. No. 7,896,105 to Dupriest discloses a method of drilling and producing hydrocarbons from subsurface formations. The method includes identifying a field having hydrocarbons. One or more wells are drilled to a subsurface location in the field to provide fluid flow paths for hydrocarbons to a production facility. The drilling is performed by estimating a drill rate for one of the wells, determining a difference between the estimated drill rate and an actual drill rate, and obtaining MSE data and other measured data during the drilling of the one of the wells. The MSE data and other measured data are used to determine one of a plurality of limiters that limit the drill rate. Drilling operations are adjusted to mitigate one of the plurality of limiters. The process is iteratively repeated until the subsurface formation has been reached by the drilling operations.
Although MSE is useful as a diagnostic tool for identifying and mitigating dysfunction and limiters, it is often prone to large fluctuations induced by transients and noise associated with the drilling process. By definition, MSE is an instantaneous quantity that requires an instantaneous knowledge of weight on bit (WOB), surface (or possibly down hole) torque (TQ) and depth of cut (DOC), wherein DOC is a function of the drill string rotary speed (RPM) and a rate of penetration (ROP). Quantities such as TQ, RPM and (to a slightly lesser extent) WOB can be reliably evaluated almost instantaneously using surface data.